The present invention relates to an improved method for preparing molten glass compositions. Heretofore, molten glass compositions have been prepared by charging a formulation of batch ingredients, containing all constituents desired in the glass composition, into a melter or furnace wherein heat is applied in sufficient quantity to cause the batch ingredients to fuse and react with each other forming a mass of molten constituents. The resulting molten mass is generally homogenized or refined within the melting apparatus forming a molten glass composition from which glass products are produced.
In the mass production of glass products, preparation of molten glass is preferably a continuous process wherein the melting unit contains a reservoir of molten constituents into which the batch formulation is charged at a rate responsive to the molten glass withdrawal or pull rate. During the time the molten constituents reside within the melter they further react with one another forming a homogenized molten glass of the desired composition. A typical constituent residence time for a melter producing 150 tons of molten glass per day is approximately 24 to 48 hours or greater. The temperature of the molten constituents residing in the melter must be maintained at a sufficient level to react the incoming batch ingredients. Hence, a portion of the molten constituents which volatilize at temperatures below the established melter operating temperature are lost as flue gas. Therefore, to obtain a given molten glass composition, suitable for forming, the batch formulation must compensate for the proportion of constituents lost by volatilization during melter residence.
The prior glass melting methods restrict glass compositions to compositions containing only those constituents which can survive the environment of the glass melting apparatus. By currently known methods of preparing molten glass, having particular properties or forming characteristics, it is necessary to introduce into the melting furnace all of the constituents that must be present in the final molten glass composition to produce those properties or characteristics. Many times the presence of those constituents in the melter is undesirable because of their volatility, corrosiveness, or environmental affects. The consequence, recognized for years is that the compositions of some glasses are impractical to produce commercially because of the volatility, corrosiveness or environmental affect of their constituents.
Certain hostile glasses, however, have desirable properties and are commercially produced under such adverse conditions as to add significantly to their cost. For example, borosilicate glass compositions generally contain volatile constituents such as boric oxide (B.sub.2 O.sub.3), fluorine (F.sub.2), and sodium borate Na.sub.2 O XB.sub.2 O.sub.3). These constituents not only volatilize at melter operating temperatures but also shorten melter life by their chemical attack upon refractory materials.